When a gas is to be distributed and dispersed in a liquid the common means of creating the dispersion fall into the following categories:
a) injection through circular orifices; PA1 b) injection through porous plugs; PA1 c) injection through rotating spargers; and PA1 d) injection with jet pumps, also known as venturi eductors or sometimes two-material injectors.
Each means has its own particular characteristic distribution of gas which depends on the initial size of the bubbles or droplets created at the point of injection and on the amount of coalescence occurring in the system. The tendency of a dispersion to coalesce is related to the physical properties of the two fluids and can be assisted by bulk fluid motion which brings droplets or bubbles together or gives rise to localized regions of lower pressure (cavitating) which assist the coalescence of the droplets or bubbles. In fact particularly with gas injection through a single orifice, porous plug or slot, creation of the dispersion itself results in bulk motion of the fluid which results in coalescence. Injection through rotating spargers or jet pumps is the usual means to counter this coalescence. Such devices provide strong mechanical agitation of the two phases and this results in a high degree of bulk fluid motion that can shear the coalesced bubbles or droplets into smaller sizes. In addition, the bulk fluid motion distributes the dispersed bubbles or droplets thereby decreasing the local density of the distributed phase and thus lowering the likelihood of the bubble-bubble interaction or the droplet-droplet interaction which leads to coalescence. Nevertheless, the resulting bulk motion causes localized lower pressure regions which results in coalescence and produces a wide range of bubble sizes. Thus a limitation of existing technology is that it does not produce very fine distributions of gas bubbles of a narrow size range. A further disadvantage of jet pump distribution techniques is that a large fraction of the bulk of the liquid is used in the pump to create the dispersion and thus very large amounts of energy are consumed.